The present invention relates to an apparatus for measuring the eccentricity of a rod of plastic material, in particular of a plastic sleeve enclosing a conductor, which sleeve is extruded from an extruder onto the conductor and is vulcanized in a pressure tube connected to the extruder.
It has been common practice e.g. to enclose the conductor of a cable in a plastic sleeve by means of an extruder, with the cable thereafter being introduced into a tubular housing for interlacing or vulcanizing thereof. Such apparatusses have become known from German printed application 28 26 857 or European patent application 00 99 993.
The sleeve must be of a predetermined thickness in order to obtain e.g. desired insulation characteristics. Accordingly, it has become known to provide an optical inspection device on the tubular housing for measuring the diameter of the cable. This is achieved e.g. by means of a transverse tube carrying an inspection glass of a predetermined glass composition, which allows to measure the diameter of the rod in the housing e.g. by means of an optical measuring device. In the tubular housing there is provided a light source, preferably an infrared light source for producing a sufficient background brightness for enabling a relatively precise diameter inspection by means of diode lines or the like. Such an apparatus has also become known from the above mentioned European patent application 00 99 993. If the measured diameter is not of a desired value, the speed of the extruder screw or the rod withdrawal velocity is adjusted so as to obtain the desired value.
On the one hand the diameter should exceed a desired minimum diameter, on the other hand a too big diameter is not desirable to avoid excess material consumption. Furthermore eccentricities of the sleeves may exist, which eccentricities cannot be detected by diameter measurements. If the optical indicating device indicates a desired diameter value, this does not necessarily mean that the sleeve is concentric to the conductor. If the sleeve is of eccentric shape, the cable does not have the desired insulation characteristics because the sleeve is not of sufficient thickness in certain areas.
For the above reasons the knowledge of the position of the conductor within the sleeve at the extruding process is a further important information. If measurements are taken downstream of the vulcanizing path, a correction by corresponding adjustments of the extruder tools may be too late; a substantial cable length may then be of an unacceptable eccentric shape.
From U.S. Pat. No. 4,086,044 it has become known to detect an eccentricity by induction measuring means. Inductive sensors are used to sense the position of the conductor relative to the sensors, while optical means are used to determine the position of the cable sleeve. This allows to measure or compute the wall thickness of the sleeve. Eccentricity measurements by means of inductive sensors including coils within the vulcanizing pressure tube is not practical. In particular sophisticated temperatures compensating means would be necessary to obtain at least approximately precise values.
Similar problems are encountered at the production of plastic tubes, where it is intended to obtain a circumferentially uniform wall thickness and to avoid eccentricities.